The present invention relates to a method for manufacturing a casting of light metal, particularly a aluminum alloy.
With the advent of the age of light-weight machines and tools, the demand for a casting of a light alloy is increasing, and the particular light alloy casting is currently mass-produced which requires the use of a destructible core during the manufacture thereof for the definition of a complemental aperture or cavity or any other trace complemental to the core used. In general, the temperature of the light alloy in a molten state during the pouring thereof into a mold assembly is relatively low as compared with the temperature used during the iron casting and, therefore, the destructible core tends to exhibit insufficient destructivity. In view of this, the manufacture of a light alloy casting using a destructible core poses a problem in that a relatively large amount of time-consuming and complicated labor is required to completely remove the "debris" or fragments of sands which has been used as a material for the destructible core. This substantially hampers the efficiency of mass-production of such alloys.
By way of example, where a cylinder head for an automobile internal combustion engine is to be cast using an aluminum alloy, the typical conventional method comprises, as shown in FIG. 1 of the accompanying drawings, the steps of pouring a molten aluminum alloy into a mold assembly, having therein a shaped core made of sands by the use of a resin binder such as a urea resin binder, and removing the resultant casting from the mold assembly in the form as containing the core after the solidification thereof, shaking the casting with the core therein by the application of vibrations thereto to destroy the core so that approximately half the total amount of the sands used to form the core can be removed from the casting, heating the casting to burn the resin component remaining inside the casting, either shaking or rapidly cooling the casting to allow the remaining sands to be completely flushed out of the casting, and removing the fins from the exterior of the casting. The casting so formed is subsequently heated for quenching and finally machined to make up the engine cylinder head.
According to the prior art, it is usual that the process A' including the pouring step to the fin removal step, the process B' including the heat-treatment for quenching, and the process C' including the machining step are distinctly divided to permit them to be performed in and by different divisions of labor in a factory and, therefore, it is the usual practice to deliver from the factory division performing the process A to the factory division performing the process B', the casting from which the sands and the fins have been removed completely.
In view of the above, the conventional casting method as a whole requires the heat treatment to be effected two times, one for burning the resin component remaining inside the casting during the process A' and the other for quenching during the process B'. This prior art method substantially doubles sites of heat treatment, the number of machines and equipment, the number of attendant workers and the amount of resources necessary to perform the heat treatment; thus increasing the cost of manufacture.
Since the process A', B' and C' are distinctly divided, no one of these processes have can be mingled with another one.
Apart from the above described conventional casting method, a similar casting method is well known wherein a core made of a material including salt added with water glass is used and wherein the core so made is dissolved for the removal thereof from the casting by either immersing the casting into water or pouring water into the casting. Not only is this method costly, but also, because of the salt used, the equipment as well as the resultant casting are susceptible to corrosion and, in order to avoid the possibility of being corroded, a flushing must be carried out for a substantial period of time to remove any trace of salt.